"Subsequent work has confirmed that leptin has a pleiotrophic role on the immune response and can rightly be considered, both structurally and functionally, as a proinflammatory cytokine." http://www.ncbi.nlm.nih.gov/pubmed/16929139 (Note: Pleiotrophic means one gene that influence many, so leptin can influence lots of different things.)
Also as a side note- in a very small study, leptin was tied to greater fatigue in CFS patients: "Daily cytokine fluctuations, driven by leptin, are associated with fatigue severity in chronic fatigue syndrome: evidence of inflammatory pathology" http://www.translational-medicine.com/content/11/1/93 Also, "In conclusion, there is an increasing evidence that leptin, besides its central effects on food intake and energy expenditure, is involved (per se or by synergistic action with other cytokines) in the pathogenesis of inflammatory and autoimmune diseases. The evidence that leptin signaling deficiency impairs humoral and cellular immune responses, and attenuates experimental inflammation models, supports the notions that a strategy focused on the block of leptin activity on peripheral cells might have intriguing therapeutic benefits." http://www.febsletters.org/article/S0014-5793(04)01389-4/fulltext
So, leptin can be proinflammatory and influence many different responses in the body (including energy/fatigue and immune response!). From previous studies we have looked at, we have seen the role of the proinflammatory cytokines in endometriosis. Now let's look at leptin's role in endometriosis:
"Leptin is known as the protein primarily released by fat cells and it acts in an endocrine fashion via the leptin receptor in the hypothalamus to regulate satiety status. Recently, the role of leptin has become more and more versatile after the discovery of leptin and its receptor in peripheral tissues including ovary, uterus and placenta (Cioffi et al., 1997; Karlsson et al., 1997; Masuzaki et al., 1997; Alfer et al., 2000; Gonzalez et al., 2000a; Kitawaki et al., 2000). Leptin has been reported to exert immunoregulatory, proinflammatory, mitogenic and angiogenic effects in several tissues (Gainsford et al., 1996; Wolf et al., 1999; Caprio et al., 2001). This makes it a potential candidate for contributing to the progress of endometriosis. A recent report even demonstrated that leptin levels in peritoneal fluid and serum of patients with pelvic endometriosis are increased (Matarese et al., 2000). However, the cellular origin and mechanism by which leptin modulates the formation of endometriosis is not clear. We herein present evidence showing that leptin and its receptor are differentially expressed in endometriosis and are involved in the proliferation of endometrial stromal cells....
"...leptin stimulated a significant increase in eutopic as well as ectopic endometrial stromal cell proliferation. However, this mitogenic effect of leptin was somewhat different in eutopic endometrial stromal cells compared with ectopic endometriotic stromal cells. In eutopic endometrial stromal cells, leptin caused a greater extent of cell proliferation and at much lower doses (Figure 8A). In stromal cells obtained from ectopic endometriotic implants, only high doses of leptin (3 and 10 ng/ml) induced cell proliferation and the induction was less pronounced (Figure 8B)....
"...we showed that both leptin transcripts and protein are highly expressed in ectopic endometriotic lesions. In eutopic endometrium, leptin was not detected in a half of the samples and only extremely low amounts of leptin were detected in the other half of the endometria. In concordance with our finding, contradictory reports have shown either positive or negative leptin expression in normal human endometrium (Alfer et al., 2000; Gonzalez et al., 2000a; Kitawaki et al., 2000). The reasons for differences in leptin transcript expression in eutopic endometrium are not known. Nevertheless, leptin was highly expressed in ectopic endometriotic lesions. The elevation of leptin in ectopic endometriosis was not due to differences in the stages of menstrual cycles or body mass as evidenced by marked increase of leptin in ectopic endometriotic tissues as compared to the eutopic endometrium collected from the same patients (n = 4). In addition, the mean BMI was not different between eutopic and endometriosis groups. Thus, elevated expression of leptin in ectopic endometriotic tissues may reflect the distinct biochemical nature of endometriotic lesions. Whether this is causal or consequent to the pathogenesis of pelvic endometriosis remains to be determined.
"Our result showing that leptin is markedly expressed in ectopic endometriotic lesions supports previous reports that the peritoneal fluid concentration of leptin was increased in women with endometriosis (Matarese et al., 2000; De Placido et al., 2001). Furthermore, peritoneal macrophages purified from patients with peritoneal endometriosis, ovarian endometrioma, or without any pelvic implants did not express leptin as analysed by RT–PCR (n = 6 per group, data not shown), indicating that the ectopic endometriotic lesion may have an important contribution to the elevated leptin concentration in peritoneal fluid. However, the contribution made by peritoneal fat cells should not be excluded and warrants further investigation. One report (De Placido et al., 2001), showing that peritoneal fluid concentrations of leptin are elevated in patients with peritoneal endometriosis but not ovarian endometriosis, has raised questions regarding which determinants cause the differences. It is hypothesized that these two kinds of endometrial lesions may have distinct leptin biosynthesizing capacity. In that particular report, the cellular origin of leptin contributing to the peritoneal fluid was not examined. In the present report, we have detected the leptin transcript and protein in both peritoneal implants and ovarian endometrioma and the quantity of leptin transcripts was not different between these two groups. Thus, it appears that a distinct leptin biosynthesis capacity may not be the determinant leading to differential leptin concentrations in peritoneal fluid with these two kinds of endometriosis. Alternatively, the authors suggested that a possible explanation is that leptin may be sequestrated into the cystic fluid of the endometrioma instead of being diffused into peritoneal fluid. This seems likely given our current result showing that similar amounts of leptin were produced. However, more direct evidence is needed before any conclusion can be drawn.
"To address whether the elevation of leptin expression in endometriotic implants would be involved in development of endometriosis via autocrine/paracrine mechanisms, we next characterized the expression profile of the leptin receptor in eutopic as well as ectopic endometriotic implants. Immunohistostaining showed that OB-R is expressed in both stromal and epithelial cells in eutopic endometrium. In ectopic endometriotic implants, OB-R was also positively stained....Furthermore, a recent work showed that progesterone but not estradiol reduced OB-RL expression in endometrium (Koshib et al., 2001), indicating that high concentrations of progesterone in the luteal phase may suppress the expression of OB-RL. The reasons for the discrepancies between these studies are not clear since different kinds of antibodies and/or quantification methods were used. Thus, the expression pattern of OB-R during the menstrual cycle cannot be definitely concluded at this point. The expression of the OB-R transcript in this study was inversely related to the expression pattern of leptin and the severity of endometriosis. We believe that this is due, at least in part, to the down-regulation effect of leptin on its own receptor. Indeed, the in-vitro study using eutopic endometrial stromal cells demonstrated that leptin dose-dependently inhibited the mRNA for its receptor (both the long form and total forms of leptin receptor). The physiological significance of receptor down-regulation by its homologous ligand is a safeguarding system that prevents overstimulation by the ligand, as has been reported for prostaglandin F2α and its receptor (Tsai and Wiltbank, 1998a; Tsai et al., 1998). Whether this down-regulation of OB-R by leptin plays any significant role in disease progression remains an open question and requires further investigation.
"...One of the examples is the acquisition of estrogen-producing ability in ectopic endometriotic implants (Noble et al., 1996; Bulun et al., 1999, 2000). Our recent data have also indicated that ectopic endometriotic cells of early endometriosis express high quantities of steroidogenic acute regulatory protein and produce high levels of progesterone (Tsai et al., 2001c). As a consequence, the ectopic endometriotic tissues become independent of the survival factors generated from gonads, and proliferate continuously throughout the cycle.
So leptin (similar to the effects of estrogen) causes the endo lesions to proliferate. Not only that, but leptin has a more pronounced affect on endo lesions than it does on the lining of our uterus. Also of note, is that progesterone seems to reduce the expression of leptin receptors (that's that OB-R you see in the study) in endo lesions (so this may be why progestin/progesterone may help with endo pain). In addition, it is noted from previous studies the endo lesions acquire the ability to produce their own estrogen and progesterone! So even if you have had ovaries removed, endo lesions left behind can still thrive as they can produce their own hormones. Weird little buggers!
Interestingly, only a few studies so far have evaluated leptin receptor gene and/or protein expression in endometrial tissue of women with endometriosis [16–18]. Lima-Couy et al.  evaluated the three isoforms of leptin receptor—total (OB-RT), long (OB-RL), and short (HuB219.3)—in the eutopic endometrium of patients with moderate and severe endometriosis. Those authors observed increased receptor expression in the period corresponding to embryo implantation, with no difference between patients and controls. Some authors [17, 19] have reported expression of leptin receptor in both eutopic and ectopic endometria.
"Therefore, the aims of the present study were (a) to assess leptin and OB-RL gene expression in ectopic and eutopic endometria of women with endometriosis and in eutopic endometrium of non-endometriosis controls, (b) to determine the leptin/BMI ratio in serum and PF in both groups, (c) to assess the immunoreactive presence of OB-RL in endometrium and endometriotic implants, and (d) to investigate the relationship among these variables....Conclusions: The present data suggest that serum leptin/BMI ratio is associated with the presence of endometriosis. Nevertheless, the clinical applicability of the leptin/BMI ratio for prediction of endometriosis still requires confirmation. Moreover, the increased expression of leptin and OB-RL in ectopic endometrium suggests a modulatory interaction between leptin and its active receptor and a role of leptin, an inflammatory and angiogenic cytokine, in the initiation or development of endometrial implants." http://www.hindawi.com/journals/ogi/2013/879618/
"Increased leptin expression in endometriosis cells is associated with endometrial stromal cell proliferation and leptin gene up-regulation. Abstract: Endometriosis is a polygenic disease with complex, multifactorial aetiologies affecting approximately 10% of women of reproductive age. Leptin is the product of the ob gene, which is related to reproductive function and immunological alteration. The angiogenic and mitogenic action of leptin may influence the formation of endometriosis. This study was aimed at determining whether leptin and leptin receptor expression differs in eutopic and ectopic endometria collected from laparoscopy and at investigating the pathophysiological role of leptin in the development of endometriosis. Leptin mRNA was undetectable in seven out of 14 eutopic endometria and only a minute amount was detected in the remaining samples. In contrast, there was a marked increase in leptin mRNA and protein expression in ectopic endometriotic lesions of patients with endometriosis (P < 0.05). Receptors for leptin were immunologically stained in eutopic endometrium as well as in ectopic endometriotic implants. However, the levels of mRNA for the long and total forms of leptin receptors were suppressed in association with the severity of endometriosis (P < 0.05). Administration of leptin stimulated its own mRNA expression in ectopic endometriotic stromal cells but decreased steady-state concentrations of mRNA encoding for leptin receptor (n = 6). In addition, leptin significantly enhanced both eutopic and ectopic endometrial stromal cell proliferation (P < 0.05). In conclusion, the differential distribution of mRNA for leptin and its receptor suggests an important autocrine and paracrine role for leptin in human endometriosis. The mitogenic and auto-augmentation effects of leptin may further contribute to the pathogenesis of endometriosis." http://www.ncbi.nlm.nih.gov/pubmed/11994543
"Leptin receptor is induced in endometriosis and leptin stimulates the growth of endometriotic epithelial cells through the JAK2/STAT3 and ERK pathways. Leptin acts as a potential growth stimulator in several normal and neoplastic cells. Recent studies have shown the presence of increased levels of leptin in the peritoneal fluid of patients with endometriosis, implicating leptin in the pathogenesis of endometriosis. However, the specific function of leptin in the induction of mitogenesis in endometriosis is not known. This study investigated the expression of the leptin receptor (ObR) in endometrioma tissues and immortalized endometriotic cells, and the effect of leptin on cell growth. ObR expression was higher in endometriomas than in the normal endometrium, and it was detected in 74% of epithelial and 30% of stromal endometrioma tissues. In addition, human endometriotic epithelial cells (11Z and 12Z) showed a high level of ObR when compared with endometrial cells and endometriotic stromal cells (22B). Furthermore, leptin treatment stimulated the growth of 11Z and 12Z cells, but not that of 22B cells. Knockdown of the ObR in 11Z and 12Z cells impaired the ability of leptin to induce cell growth. Leptin induced the activation of Janus Kinases 2 (JAK2), signal transducers and activators of transcription 3 (STAT3) and extracellular signal-regulated kinase (ERK) in endometriotic epithelial cells. Moreover, pretreatment with the JAK2/STAT3 inhibitor AG490 and the ERK inhibitor PD98059 significantly inhibited leptin-induced cell growth. The present results show that the ObR is induced in endometriosis, and that leptin stimulates the growth of endometriotic epithelial cells through the JAK2/STAT3 and ERK pathways." http://www.ncbi.nlm.nih.gov/pubmed/23184927